Method and system for generating a ramping signal

ABSTRACT

A system is provided for generating a ramping signal. The system includes a plurality of storage circuits each including an input and an output. The output of a previous storage circuit is connected to the input of a next storage circuit. The storage circuits are configured to propagate a first enable signal based on a first control signal. The system also includes a plurality of first current generating circuits. Each first current generating circuit is coupled to the output of a corresponding storage circuit to receive the propagated first enable signal. The first current generating circuits are configured to generate a first current signal based on the propagated first enable signal.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Non-provisional patentapplication Ser. No. 14/560,371, filed with the United States Patent andTrademark Office on Dec. 4, 2014, and entitled “METHOD AND SYSTEM FORGENERATING A RAMPING SIGNAL,” which is hereby incorporated by referencein its entirety. The U.S. Non-provisional patent application Ser. No.14/560,371 claims the benefit of priority to U.S. ProvisionalApplication No. 61/915,444, filed with the United States Patent andTrademark Office on Dec. 12, 2013, and entitled “A METHOD TO GENERATE AMONOTONIC RAMPING SIGNAL WITH LATCH PROPAGATED ENABLE SIGNAL,” which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to methods and circuits for image sensingapplications, in particular, to methods and circuits for generating aramping signal as a reference signal for image sensor circuits.

BACKGROUND INFORMATION

Digital cameras, scanners, and other imaging devices often use imagesensors, such as charge-coupled device (CCD) image sensors orcomplementary metal-oxide-semiconductor (CMOS) image sensors, to convertoptical signals to electrical signals. An image sensor typicallyincludes a grid of pixels, row access circuitry, column accesscircuitry, and a ramp signal generator. The pixels capture the lightimpinged on them and convert the light signals to electrical signals.The row access circuitry controls which row of pixels that the sensorwill read. The column access circuitry includes column read circuitsthat read the signals from corresponding columns. The ramp signalgenerator generates a ramping signal as a global reference signal forcolumn read circuits to record the converted electrical signal. Inoperation, the quality of the ramping signal can significantly affectthe quality of the output of the image sensor. For example, a rampingsignal with poor linearity can cause a gain non-linearity of the columnread circuits. In addition, a ramping signal with a large glitch powercan have a lost-bit effect. Moreover, a ramp signal generator with lowpower consumption and a small physical area is often desired.

SUMMARY

The present disclosure provides a system for generating a rampingsignal. According to some embodiments, the system includes a pluralityof storage circuits each including an input and an output. The output ofa previous storage circuit is connected to the input of a next storagecircuit. The storage circuits are configured to propagate a first enablesignal based on a first control signal. The system also includes aplurality of first current generating circuits. Each first currentgenerating circuit is coupled to the output of a corresponding storagecircuit to receive the propagated first enable signal. The first currentgenerating circuits are configured to generate a first current signalbased on the propagated first enable signal.

The present disclosure also provides a method for generating a rampingsignal. According to some embodiments, the method includes applying afirst enable signal to a series of storage circuits, each of the storagecircuits including an input and an output, the output of a previousstorage circuit being coupled to the input of a next storage circuit;applying a clock signal to the series of storage circuits, wherein theclock signal enables the series of storage circuits to propagate thefirst enable signal through the series of storage circuits; andgenerating a first current signal based on the propagated first enablesignal, wherein the first current signal increases every time when thefirst enable signal propagates through a storage circuit.

The present disclosure further provides a system for generating aramping signal. The system includes a plurality of latches connected inseries and configured to propagate a first enable signal based on aclock signal, and a plurality of first current generating circuitscoupled to outputs of the latches. The first current generating circuitsare configured to generate a first current signal based on thepropagated first enable signal. The system further includes one or moresecond current generating circuits configured to generate a secondcurrent signal and a load block coupled to the plurality of firstcurrent generating circuits and the one or more second currentgenerating circuits. The load block converts the first and secondcurrent signal to a voltage signal to generate the ramping signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary image sensor comprising anexemplary embodiment of a pixel grid and an exemplary embodiment ofcolumn read circuits.

FIG. 2 is a block diagram illustrating an exemplary ramp signalgenerator.

FIG. 3 is a block diagram illustrating an exemplary current generatingcircuit block comprising an exemplary embodiment of a ramp segment andan exemplary embodiment of an offset segment.

FIG. 4 is a more detailed block diagram illustrating an exemplary rampsegment, an exemplary offset segment, and exemplary load circuitscorresponding to those shown in FIG. 2.

FIG. 5 is a block diagram of an exemplary gated or clocked D latch.

FIG. 6 is a schematic diagram of an exemplary embodiment of a currentgenerating circuit.

FIG. 7A is a diagram illustrating an exemplary voltage-counter valuerelation of a first component of the ramping signal corresponding to aramp current signal generated by the exemplary ramp segment shown inFIGS. 2A and 2B.

FIG. 7B is a diagram illustrating an exemplary voltage-counter valuerelation of a second component of the ramping signal corresponding to anoffset current signal generated by the exemplary offset segment shown inFIGS. 2A and 2B.

FIG. 7C is a diagram illustrating an exemplary voltage-counter valuerelation of the ramping signal corresponding to the sum of the currentsignals generated by the ramp segment and the offset segment shown inFIGS. 2A and 2B.

FIG. 8 is a diagram illustrating exemplary voltage-time relations of aplurality of ramping signals.

FIG. 9 is a flowchart representing an exemplary method for generating aramping signal.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference will now be made in detail to the exemplary embodimentsconsistent with the embodiments disclosed herein and the examples ofwhich are illustrated in the accompanying drawings. Wherever possible,the same reference numbers will be used throughout the drawings to referto the same or similar parts.

FIG. 1 is a diagram illustrating an exemplary image sensor 100. Imagesensor 100 can be a CMOS type image sensor or a CCD type image sensor,or any other type of image sensors. Image sensor 100 may include atwo-dimensional grid of pixels 102, row access circuitry 103, columnaccess circuitry 104, a ramp signal generator 106, a series ofcomparators 107A-N, a storage block 108, and a control block 109. Theimage sensor circuitry is divided into blocks as shown in FIG. 1 forillustration purpose. A person having ordinary skill in the art shouldunderstand that at least some of these blocks can be integrated togetheron one chip.

Pixel grid 102 includes multiple pixels for sensing light signals andconverting the light signals to electrical signals. Each pixel cangenerate a voltage that is proportional to the energy of the sensedlight signal. As shown in FIG. 1, pixel grid 102 is electrically coupledto row access circuitry 103 and column access circuitry 104. Rowaccessing circuitry 103 can select one row at a time in pixel grid 102.When a particular row is selected, voltages generated by the pixels inthat row can become accessible by, for example, column access circuitry104.

Column access circuitry 104 includes multiple column read circuits (notshown) each corresponding and being coupled to a column in pixel grid102. The column read circuits read the voltage signals generated by thepixels in the corresponding columns and provide output readout signals.

Ramp signal generator 106 generates a ramping signal, e.g., amonotonically increasing or decreasing voltage signal “Vramp.” Theinternal circuits of ramp signal generator 106 will be described indetail below. The ramp signal generator 106 is described in the contextof an image sensor. A person having ordinary skill in the art shouldappreciate that the disclosed ramp signal generator and the rampingsignal can be used in other devices.

Comparators 107A-N each corresponds and is electrically coupled to oneof the column read circuits to receive the readout signal. Comparators107A-N are also electrically coupled to ramp signal generator 106 toreceive the Vramp signal. Each comparator (e.g., 107A) compares thereadout signal with the Vramp signal. The term “compare” is used in abroad sense in this application. For example, one way to compare the twosignals is coupling the two signals to two input terminals of acomparator. Another way is to combine the two signals (or variations ofthe two signals) and compare the combined signal with a referencesignal. The reference signal can be a reference voltage or current. Insome embodiments, the reference signal can be a threshold voltage of atransistor, and when the combined signal reaches the reference signallevel, it will turn on or off the transistor. Because the value of thereference signal and ramping signal can be known, the value of the inputsignal can be derived. A person having ordinary skill in the art shouldbe able to design other ways to derive the value of the input signal. Insome embodiments, the Vramp signal may start from zero and monotonicallyincrease (or start with a maximum value and monotonically decrease).When Vramp changes from lower than the readout signal to higher than thereadout signal, the comparator output will switch from, e.g., low tohigh (e.g., from 0 to 1). Alternatively, the readout signal and theVramp signal, or variations of these signals, can be combined and whenthe combined signal reaches a certain level, the comparator output willswitch.

As shown in FIG. 1, the comparators 107A-N are electrically coupled to astorage block 108. Storage block 108 can include, for example, staticrandom access memory (SRAM), dynamic random access memory (DRAM), flashmemory, or any other type of storage circuits that can store analog ordigital signals.

Control block 109 is electrically coupled to row access circuitry 103,column access circuitry 104, ramp signal generator 106, comparators107A-N, and storage block 108, and provides one or more signals forcontrolling these circuits. For example, control block 109 can providecontrol signals to row access circuitry 103 for controlling the speed ofaccessing the rows in pixel grid 102. Control block 109 can alsogenerate any desired signals for controlling other circuits in imagesensor 100.

Control block 109 may include one or more counters that are electricallycoupled to comparators 107A-N and storage block 108. For example, eachcounter may be coupled to a corresponding comparator 107. Control block109 may also generate a clock signal to trigger the counter to count theclock cycles. The comparators 107A-N compare Vramp and the readoutsignals from the columns of pixel grid 102. At the comparator, forexample, comparator 107A, when Vramp becomes higher than the readoutsignal from column A, or the combination of Vramp and the readout signalreaches to a certain level, comparator 107A switches its output from 0to 1, and that will trigger storage block 108 to record the value in thecounter corresponding to comparator 107A at that moment. The recordedvalue can be a digital representation of the readout signal from columnA.

As discussed above, row accessing circuitry 103 can select one row at atime in pixel grid 102 and column access circuitry 104 can read thevoltage signals generated by the pixels in the selected row. The voltagesignals obtained by column access circuitry 104 are converted to digitalsignals and stored in storage block 108. After all rows are selected andall voltage signals are readout, the image sensed by pixel grid 102 canbe represented by a two-dimensional grid of digital representationsstored in storage block 108 and that can be used to form a digitalimage.

FIG. 2 is a block diagram illustrating an exemplary ramp signalgenerator 106. Ramp signal generator 106 may include, among otherthings, a current generating circuit block 120, a load block 140, and acontrol circuit block 160. As shown in FIG. 2, these circuits areelectrically coupled to each other.

Current generating circuit block 120 may include one or more currentgenerating segments, for example, one or more ramp segments 150 and oneor more offset segments 180. A ramp segment 150 can include one or morecurrent generating circuits that generate a ramp current signal. Anoffset segment 180 can include one or more current generating circuitsthat generate an offset current signal. In some embodiments, the rampcurrent signal, in conjunction with load block 140, provides a firstcomponent of the ramping signal Vramp 110, and the offset currentsignal, in conjunction with load block 140, provides a second componentof the ramping signal Vramp 110. In some embodiments, only the rampcurrent signal generated by ramp segment 150 is used to generate rampingsignal Vramp 110.

In some embodiments, ramp signal generator 106 has a generallylongitudinal shape that extends along an edge of pixel grid 102, forexample, in a row direction as shown in FIG. 1. Within ramp signalgenerator 106, the ramp segment 150 and the offset segment 180 can belogically evenly distributed. In some embodiments, the circuits in theramp segment 150 and the offset segment 180 can be physicallydistributed in the longitudinal direction. This layout of ramp signalgenerator 106 can help save the overall space of the image sensor. Inaddition, this layout can also help place ramp signal generator 106close to column access circuitry 104 and comparators 107A-N, as shown inFIG. 1. This arrangement can help reduce voltage drop from the output ofthe ramp signal generator 106 to the input of comparators 107A-N. Aperson having ordinary skill in the art should understand that rampsegment 150 and offset segment 180 can be physically distributed in anyother desired manner.

Moreover, by physically placing ramp signal generator 106 in closeproximity to the corresponding column access circuitry 104, thenon-linearity or glitch power of the ramping signal Vramp 110 can alsobe reduced or minimized; the layout shape of the image sensor 100 canhave more flexibility; and the sensing speed of the image sensor 100 canalso be improved.

Load block 140 may include, for example, resistive, capacitive, and/orinductive loads for generating ramping signal Vramp 110 based on theramp current signal generated by ramp segment 150 and/or the offsetcurrent signal generated by offset segment 180. For example, load block140 may include one or more resistors. The currents generated by rampsegment 150 and offset segment 180 pass through the resistors togenerate the ramping voltage signal Vramp 110.

Control circuit block 160 can be electrically coupled to control block109 of the image sensor 100 to receive one or more control signals 130.In some embodiments, control circuit block 160 can be a portion ofcontrol block 109. Control signal 130 can include any number of controlsignals in analog or digital domain. Based on control signal 130,control circuit block 160 can generate a control signal 170 forcontrolling current generating circuit block 120. Control signal 170 caninclude any number of control signals in analog or digital domain, andcan provide, for example, a clock signal, a ramp enable signal, and/orany control signals for the operation of current generating circuitblock 120. Control circuit block 160 can also provide a control signal190 for controlling load block 140. Similarly, control signal 190 caninclude any number of control signals in analog or digital domain.Control signal 190 configures the load in load block 140. For example,control signal 190 can adjust the total equivalent resistance load ofload block 140 by turning on or off of one or more resistors. In someembodiments, the total equivalent resistance load can be adjusted in abinary weighted manner (e.g., adjusting the resistance load by abinary-coded control signal). In some embodiments, the total equivalentresistance load can be adjusted in thermometer code manner (e.g.,adjusting the resistance load by a thermometer coded control signal), orin any other desired manner. It should be appreciated that controlcircuit block 160 can include any logic that is suitable for generatingcontrol signals 170 and 190 based on control signal 130. For example,control circuit block 160 can include combinational logics and/orsequential logics.

FIG. 3 is a block diagram illustrating an exemplary embodiment ofcurrent generating circuit block 120. As shown in FIG. 3, in someembodiments, ramp segment 150 can include a storage circuit array 202and a ramp current generating cell block 210. Storage circuit array 202can include one or more storage circuits 290A-N (collectively referredto as storage circuits 290) connected in a chain such that an output ofa previous storage circuit is electrically coupled to an input of a nextstorage circuit. For example, the output of storage circuit 290A iselectrically coupled to storage circuit 290B and similarly, the outputof storage circuit 290B is electrically coupled to storage circuit 290C.

Storage circuits 290 can be any circuits that are capable of storing oneor more stable logic states (e.g., “0” or “1”). Storage circuits 290 caninclude, for example, latches or flip-flops. As an example, storagecircuits 290 can be gated or clocked D latches as shown in FIG. 5. Eachlatch has a data input terminal D, a clock or enable terminal E, and twooutput terminals. When the clock terminal E is on, the input signalreceived at the terminal D passes through the circuit, to the output Q.

Storage circuit array 202 can be electrically coupled to control circuitblock 160 to receive control signal 170. Control signal 170 may includea control signal, e.g., a clock signal 240 and a ramp enable signal 250.In some other embodiments, ramp enable signal 250 may be provided fromanother source, such as control block 109. Clock signal 240 can be adigital or analog clock signal that provides timing information (e.g., aperiodical pulse) for operating storage circuits 290A-N. Storagecircuits 290 can be single or double edge sampling circuits, and can berising or falling edge triggering circuits, e.g., edge triggeringregisters. Ramp enable signal 250 is the input of the first storagecircuit 290A in storage circuit array 202. Each of storage circuits290A-N outputs a signal 260, which is coupled to the input of the nextstorage circuit (except for the last storage circuit). In the example ofthe storage circuit as shown in FIG. 5, clock signal 240 can be coupledto the clock terminal E, ramp enable signal 250 can be coupled to thedata input terminal D, and output terminal Q outputs signal 260.

Ramp current generating cell block 210 can, for example, include one ormore ramp current generating circuits 220A-N (collectively referred toas ramp current generating circuits 220). Ramp current generatingcircuits 220A-N can be coupled to the output of corresponding storagecircuits 290A-N to receive the output signals 260A-N. Ramp currentgenerating circuits 220A-N use the output signals 260A-N as enablesignals 280A-N to enable ramp current generating circuits 220A-N togenerate ramp current signals. For example, if enable signal 280A isasserted (e.g., becomes “1” or high), ramp current generating circuit220A can be turned on to generate a ramp current signal.

Ramp current generating circuits 220 can also receive and be controlledby a bias signal 270 from, for example, control block 160. Bias signal270 can be included in control signal 170. Bias signal 270 can be avoltage or current signal providing a biasing voltage or current to rampcurrent generating circuits 220. As shown in FIG. 3, each of the rampcurrent generating circuits 220 can be individually controlled by enablesignal 280 and bias signal 270. The outputs of ramp current generatingcircuits 220 are connected to a node that generates the ramping signalVramp 110.

The operation of ramp segment 150 will be explained in conjunction withFIG. 3. The explanation is based on an assumption that storage circuits290A-N are single and rising edge triggering circuits. As discussedabove, a person having ordinary skill in the art should understand thatstorage circuits 290A-N can be falling edge triggering circuits ordouble-edge triggering circuits. When ramp enable signal 250 is assertedat the input of storage circuit 290A, and when clock signal 240 is atthe first rising edge, storage circuit 290A switches its output signal260A to high. The output signal 260A is applied to the input of storagecircuit 290B. When the next cycle of rising edge of clock signal 240arrives, storage circuit 290B switches its output signal 260B to high.The output signal 260B is then applied to the input of storage circuit290C. In this manner, ramp enable signal 250 propagates through storagecircuits 290A-N based on clock signal 240.

When output signal 260A is high, enable signal 280A is high (which isthe same signal as 260A). When enable signal 280A is high and biassignal 270 is high, it turns on ramp current generating circuit 220A togenerate a ramp current. Similarly, after the second rising edge ofclock signal 240, enable signal 280B becomes high, which in turn, turnson ramp current generating circuit 220B to generate a second rampcurrent. In this manner, as ramp enable signal 250 propagates throughstorage circuits 290A-N, enable signals 280A-N successively become high,which turn on current generating circuits 220A-N. The total ramp currentgenerated by ramp current generating circuits 220A-N are converted to avoltage signal by load block 140. The voltage signal can be the rampingsignal Vramp 110 or a component of the ramping signal Vramp 110. Theabove describes an example of generating a monotonically increasingramping signal. A person having ordinary skill in the art shouldunderstand that the circuits can be configured to generate other typesof ramping signals. For example, ramp current generating circuits 220A-Ncan be initially configured with “on” state, and enable signals can beconfigured to turn off the ramp current generating circuits 220A-N. Insuch a configuration, the ramping signal Vramp 110 will be amonotonically decreasing signal.

Referring again to FIG. 3, offset segment 180 can include an offsetcurrent generating cell block 212. Offset current generating cell block212 can, for example, include one or more offset current generatingcircuits 222A-N (collectively referred to as offset current generatingcircuits 222). In some embodiments, offset current generating circuits222 can be the same or similar to ramp current generating circuits 220.In some embodiments, offset current generating circuits 222 can bedifferent from ramp current generating circuits 220. For example, anoffset current generating circuit 222 can include transistors that havesmaller size (e.g., width and/or length) than transistors in rampcurrent generating circuits 220.

Offset current generating circuits 222 can receive enable signals 282A-Nand a bias signal 272. In some embodiments, enable signals 282A-N andbias signal 272 can be included in control signal 170 generated bycontrol circuit block 160 or any other circuits suitable for generatingsuch signals. In some embodiments, enable signals 282A-N and bias signal272 can be the same as or different from corresponding enable signal280A-N and bias signal 270, respectively. In some embodiments, each ofoffset current generating circuits 222 can be turned on or offindividually based on enable signal 282 and bias signal 272. Forexample, if enable signal 282A and bias signal 272 are asserted, offsetcurrent generating circuit 222A is turned on to generate an offsetcurrent signal. If enable signal 282B and bias signal 272 are asserted,offset current generating circuit 222B is turned on to generate anotheroffset current signal. By controlling the number of offset currentgenerating circuits 222 that are turned on, the total amount of currentgenerated by offset current generating circuits 222 can be configured inany desired manner. As shown in FIGS. 2 and 3, the currents generated byramp current generating circuits 220 and offset current generatingcircuits 222 can be combined to generate ramping signal Vramp 110.

FIG. 4 illustrates one embodiment of ramp segment 150 and offset segment180. As shown in FIG. 4, load block 140 includes a series of resistors242A-N and 244. Resistors 242A-N are coupled to ramp segments 150 andresistor 244 is coupled to offset segment 180. In certain embodiments,both resistors 242A-N and resistor 244 are coupled to ramping signalVramp 110. The ramp current generated by ramp segment 150 and the offsetcurrent generated by offset segment 180 are combined at the node, wherethe combined current is converted to the ramping voltage signal Vramp110, by the resistors 242A-N and 244. Control circuit block 160, byproviding control signal 190, can control the number of resistors thatare electrically coupled to ramp segment 150 and offset segment 180.

FIG. 6 is a schematic diagram of an exemplary embodiment of a currentgenerating circuit 300. Current generating circuit 300 can be used as,for example, a ramp current generating circuit 220 and/or an offsetcurrent generating circuit 222. In some embodiments, current generatingcircuit 300 can include a bias transistor 310 and a select transistor320. Bias transistor 310 and/or select transistor 320 can be P-typemetal-oxide-semiconductor (PMOS) transistors, N-typemetal-oxide-semiconductor (NMOS) transistors, or any other type oftransistors (e.g., bipolar transistors). The size (e.g., transistorwidth and gate length) and the type of bias transistor 310 and selecttransistor 320 can be varied. As an example, if offset currentgenerating circuit 222 generates an offset current that is smaller thanthe ramp current generated by ramp current generating circuit 220, thesize of bias transistor 310 and/or select transistor 320 in offsetcurrent generating circuit 222 can thus be smaller than the size ofcorresponding transistors in ramp current generating circuit 220.Moreover, the size of bias transistor 310 and the size of selecttransistor 320 can be the same or different. It should be understoodthat the size of bias transistor 310 and the size of select transistor320 can be configured in any desired manner.

As discussed above, in some embodiments, PMOS transistors can be used toimplement bias transistor 310 and select transistor 320. As shown inFIG. 6, when PMOS transistors are used, the source terminal of biastransistor 310 can be electrically coupled to a supply voltage 360. Thedrain terminal of bias transistor 310 can be electrically coupled to thesource terminal of select transistor 320. The drain terminal of selecttransistor 320 can be electrically coupled to, for example, load block140 to enable the generating of ramping signal Vramp 110. The gateterminal of bias transistor 310 and the gate terminal of selecttransistor 320 can be electrically coupled to bias signal 370 and enablesignal 380, respectively. In some embodiments, bias signal 370 can be adigital or analog signal providing a biasing voltage or current to biastransistor 310. Enable signal 380 can be an analog or digital signalthat turns on or off select transistor 320, such that the currentgenerated by current generating circuit 300 can be enabled or disabled.Bias signal 370 and enable signal 380 can represent, for example, biassignal 270 and enable signal 280 when current generating circuit 300represents a ramp current generating circuit 220, or bias signal 272 andenable signal 282 when current generating circuit 300 represents anoffset current generating circuit 222.

In certain embodiments, current generating circuit 300 may also includeone or more cascode transistors (not shown). The cascade transistors maybe electrically coupled to select transistor 320 and/or bias transistor310 to increase the amplifier gain.

FIG. 7A is a diagram illustrating an exemplary voltage-counter valuerelation curve 410 corresponding to the first component of rampingsignal Vramp 110 generated by the current from ramp segment 150. FIG. 7Bis a diagram illustrating an exemplary voltage-counter value relationcurve 420 corresponding to the second component of ramping signal Vramp110 generated by the current from offset segment 180. FIG. 7C is adiagram illustrating an exemplary voltage-counter value relation 430corresponding to the ramping signal Vramp 110 generated by the sum ofthe currents generated by ramp segment 150 and offset segment 180.

As shown in FIG. 7A, the vertical axis represents a voltage levelmeasured in volts, and the horizontal axis represents the value of thecounter (e.g., the counter shown in FIG. 1, which is used to record thecorresponding ramping signal) measured by the number of leastsignificant bits (LSBs). The counter is driven by a clock signal, forexample, clock signal 240 or any other clock signal, and the countervalue is associated to time. Thus, the diagrams in FIGS. 7A-C also showvoltage level and time relations. In operation, ramping signal Vramp110, as a reference signal, may need to increase from 0V to apredetermined voltage (e.g., the power supply voltage). Differentvoltage levels can be represented by different digital representationsmeasured by the number of LSBs. For example, 0V can be represented by a0 LSB, 2ΔV can be represented by 2 LSBs, and the maximum voltage can berepresented by the maximum number of LSBs (e.g., 8 LSBs).

As discussed above, when ramp enable signal 250 is asserted and clocksignal 240 is applied, ramp enable signal 250 can propagate throughstorage circuits 290A-N. As ramp enable signal 250 propagates, rampcurrent generating circuits 220A-N can be turned on one after another togenerate ramp currents. As a result, the total amount of currentgenerated by ramp current generating circuits 220 increases with theclock signal 240. The current generated by ramp current generatingcircuits 220 is applied to load block 140, which converts the current toa voltage signal. The converted voltage signal constitutes the firstcomponent of ramping signal Vramp 110, as shown in FIG. 7A.

For example, the first component of ramping signal Vramp 110,represented by 410, can start at 0V. After ramp enable signal 250propagates through storage circuit 290A, ramp current generating circuit220A can be turned on to generate a ramp current that flows through loadblock 140. As a result, the first component of ramping signal Vramp 110can increase to a voltage level of 2ΔV corresponding to 2 LSBs in thehorizontal axis. Similarly, after ramp enable signal 250 propagatesfurther, more ramp current generating circuits 220 can be turned on andramping signal Vramp 110 can increase to 4ΔV, 6ΔV, etc., correspondingto 4 LSBs, 6 LSBs, etc., respectively. As seen in FIG. 7A, the firstcomponent of ramping signal Vramp 110 can be a monotonically increasingsignal with a staircase shape. In certain embodiments, filteringcircuits (not shown) may be coupled to ramp current generating circuits220 such that the first component of ramping signal 110 is a smooth rampcurve, e.g., a curve without stairs, glitches, and/or spikes.

In some other embodiments, the first component of ramping signal Vramp110 can be a monotonically decreasing signal. For example, initially,all ramp current generating circuits 220 can be turned on and the firstcomponent of ramping signal Vramp 110 can start at the maximum voltagelevel (e.g., power supply voltage) corresponding to 8 LSBs. An invertedramp enable signal 250 can propagate through storage circuits 290A-N toturn off one or more of ramp current generating circuits 220. As aresult, the first component of ramping signal Vramp 110 can decrease to6ΔV, 4ΔV, 2ΔV, etc., corresponding to 6 LSBs, 4 LSBs, 2 LSBs, etc.

Referring to FIG. 7B, in some embodiments, offset segment 180 candynamically generate an offset current. The offset current can beconfigured to generate an offset voltage ΔV, which is the half of thestep size of the voltage generated by ramp segment 150 shown in FIG. 7A.In some embodiments, enable signals 282A-N can be clock or pulse signalsthat have the same frequency as clock signal 240. Enable signals 282A-Ncontrol offset current generating circuits 222 to generate a current ina square wave form, which can be converted to a voltage signal (secondcomponent of ramping signal Vramp 110) in a square wave form, as shownin FIG. 7B.

FIG. 7C illustrates the ramping signal Vramp 110, which is thecombination of the first component in FIG. 7A and second component inFIG. 7B. As shown in FIG. 7C, the exemplary result ramping signal 430 isa monotonically increasing signal. As shown in FIG. 7A, the step size ofthe first component of ramping signal Vramp 110 is 2ΔV. As shown in FIG.7C, after combining the first component with the second component, thestep size of ramping signal Vramp 110 is reduced to ΔV. As a result, theresolution of ramping signal Vramp 110 is increased. The system achievesthe higher resolution by using the offset segment 180. Such anarrangement reduces the design complexity without sacrificing theperformance of ramp signal generator 106. It should be appreciated thatoffset segment 180 can generate offset currents in any desired manner,such as at a frequency that is a fraction or a multiplication of thefrequency of clock signal 240. For example, storage circuits 290A-N canbe double-edge trigger circuits, and the frequency of enable signal 282of offset segment 180 can be double of the frequency of clock signal 240of ramp segment 150. In some other embodiments, the offset segment 180can be configured to generate static offset currents that do not changeover time. In some applications, which do not require a high resolution,offset segment 180 may be disabled. In some other embodiments, the rampsignal generator 106 can be implemented without an offset segment. Forexample, ramp segment 150 can be implemented with sufficient number ofstorage circuits and ramp current generating circuits to achieve adesired resolution. In this situation, ramp signal generator 106 may notneed an offset segment.

In some other embodiments, offset segment 180 can further include astorage circuit array (not shown) similar to storage circuit array 202.An offset enable signal (not shown), which can be similar to ramp enablesignal 250, can propagate through the storage circuit array associatedwith offset segment 180. In a similar manner to those described abovewith respect to the operation of ramp current generating circuits 220,one or more offset current generating circuits 222 can be turned on insequence such that the offset current increases in a staircase mannerwith a step size of ΔI. Correspondingly, the offset voltage can increasein a staircase manner with a step size of ΔV. In certain embodiments,filtering circuits (not shown) may be coupled to offset currentgenerating circuits 222 such that the offset voltage is a smooth rampcurve, e.g., a curve without stairs, glitches, and/or spikes.

The second component of ramping signal Vramp 110 (i.e., offset voltage)can be adjusted with different factors. For example, as described above,the magnitude of offset voltage can be configured by the number ofoffset current generation circuits 222. In some embodiments, themagnitude of offset voltage can also be configured by the voltage orcurrent level of bias signal 272. For example, if the voltage or currentlevel of bias signal 272 increases, the offset current generated byoffset current generation circuits 222 can increase. As a result, themagnitude of offset voltage can also increase. The magnitude of offsetvoltage can also be configured by the size of bias transistor 310 (e.g.,transistor width and gate length). For example, if the width of biastransistor 310 increases, the offset current generated by offset currentgeneration circuits 222 can increase. As a result, the magnitude ofoffset voltage can also increase.

FIG. 8 is a diagram illustrating exemplary voltage-time relation curves510A-C of ramping signal Vramp 110. In FIG. 8, the vertical axisrepresents the voltage level measured in volts; the horizontal axisrepresents the time measured in seconds. As shown in FIG. 8, rampingsignal Vramp 110 can have different voltage-time ratios and the curves510A-C can have different slopes. The slope of ramping signal Vramp 110can be important in many applications. For example, in CMOS image sensorapplications, the slope of ramping signal Vramp 110 can determine theanalog to digital signal converting factor, which can be equal to thesignal path gain. Signal path gain can be defined as the input signalswing divided by the conversion delay of analog to digital convertersused in the image sensor circuitry. A higher slope of ramping signalVramp 110 can correspond to a lower signal path gain. In particular, anincreasing slope of ramping signal Vramp 110 denotes an increasingvoltage difference within one unit time period, such as one counterclock period. As a result, an increasing slope can result in adecreasing analog to digital signal converting factor, and thus adecreasing signal path gain.

The slope can be adjusted by many factors, for example, by adjusting thecurrent generated by current generating circuits 220, the length of theramping signal Vramp 110, and the total resistance of the load block140.

In some embodiments, the current generated by current generatingcircuits 220 can be adjusted by at least one of: the voltage or currentlevel of bias signal 270; the size of bias transistor 310 (which is usedto implement current generating circuits 220); the number of rampcurrent generating circuits 220.

In some embodiments, the length of ramping signal Vramp 110 can beadjusted by changing the time or speed that enable signal 250 propagatesthrough storage circuits 290. The time or speed can be adjusted byadjusting at least one of: the frequency of clock signal 240, the lengthof time of ramp enable signal 250, and the number of storage circuits290 and ramp current generating circuits 220.

The load of load block 140 can be adjusted by adjusting the number ofload components (e.g., number of resistors) connected and/or thecapacity of each component (e.g., resistance of the resistors).

Referring back to FIG. 8, curve 510A has a slope that is greater thanthat of curve 5106, which has a slope that is greater than that of curve510C. In some embodiments, curve 510A may represent ramping signal Vramp110 that has at least one of: a larger number of ramp current generatingcircuits 220, a higher or lower biasing voltage provided by bias signal370, a larger load provided by load block 140, or a higher frequency ofclock signal 240. It should be appreciated that while curves 510A-C areshown as monotonic increasing signal (e.g., a linearly ramping signalfrom a low voltage to a high voltage), ramp signal generator 106 can beconfigured to generate ramping signal Vramp 110 that has any desiredcurve (e.g., a non-monotonic curve and/or a non-linear curve). A linearcurve is a curve that has a mathematical relationship or function thatcan be graphically represented as a straight line, where two quantitiescorresponding to the straight line are directly proportional to eachother.

FIG. 9 is a flowchart representing an exemplary method for generating aramping signal. It will be readily appreciated that the illustratedprocedure can be altered to include less or more steps. In an initialstep 610, enable signal 250 and clock signal 240 are applied to rampsignal generator 106. Ramp signal generator 106, via storage circuits290, can propagate (step 620) ramp enable signal 250 based on clocksignal 240. As shown in FIG. 9, ramp signal generator 106, via rampcurrent generating circuits 220, can generate (step 630) a ramp currentsignal based on the propagated ramp enable signal 250.

As discussed above, in some embodiments, ramp signal generator 106 mayinclude an offset segment 180 that generates an offset current. Theclock signal may be applied to offset segment 180 for controlling offsetsegment 180 to generate the offset current at step 640. At step 650, theramp current and offset current are converted to the ramping signalVramp 110, via load block 140.

In some embodiments, ramp segment 150 can generate a plurality oframping signals at any specific times. Upon generating the rampingsignals at the specific times, ramp enable signal 250 can stoppropagate. One or more of these ramping signals can generate a firstcomponent of ramping signal Vramp 110. And one or more of other rampingsignals can generate a second component (and in some embodiments, athird component, a fourth component, etc.,) of ramping signal Vramp 110.The second component of ramping signal Vramp 110 can be used as an fixedoffset signal to increase the resolution of ramping signal Vramp 110 asdiscussed above. In this manner, ramp signal generator 106 can generatea high resolution ramping signal without using offset segment 180. Aperson of ordinary skill in the art that the one ramp segment 150 cangenerate a plurality of ramping signals such that a plurality of fixedoffset signals can be obtained at different specific times as required.Moreover, in some embodiments, by generating a plurality of rampingsignals, ramp segment 150 can enable the capability of using two or moreramping signals in the analog to digital conversion (ADC) to reduce thevertical fixed pattern noise in CMOS image sensor applications. Asdescribed above, ramp segment 150 can generate a plurality of offsetsignals. These offset signals can cancel certain circuit offsets suchthat the variation of the equalized point of comparator 107 is reducedor minimized. As a result, the vertical fixed pattern noise is reduced.

In some embodiments, ramp signal generator 106 can include two rampsegments 150. The two ramp segments 150 can propagate two enable signals250 from both ends of current generating circuits 220 to minimize the IRvoltage drop across the ramping signal Vramp 110. In certainembodiments, the two ramp segments 150 can propagate two enable signals250 from the middle of current generating circuits 220 to both ends. Insome embodiments, by using at least one of ramp segment 150 and offsetsegment 180, ramping voltage signal Vramp 110 can be generated withoutusing a decoder and/or encoder circuits.

In the preceding specification, the subject matter has been describedwith reference to specific exemplary embodiments. It will, however, beevident that various modifications and changes may be made withoutdeparting from the broader spirit and scope of the subject matter as setforth in the claims that follow. The specification and drawings areaccordingly to be regarded as illustrative rather than restrictive.Other embodiments may be apparent to those skilled in the art fromconsideration of the specification and practice of the embodimentsdisclosed herein.

What is claimed is:
 1. A system for generating a ramping signal,comprising: a plurality of storage circuits each including an input andan output, the output of a previous storage circuit being connected tothe input of a next storage circuit, the storage circuits beingconfigured to propagate a first signal; and a plurality of firstcircuits, each being coupled to the output of a corresponding storagecircuit to receive the propagated first signal and to generate a secondsignal based at least on the received propagated first signal, the firstcircuits being configured to obtain a ramp current signal based on thegenerated second signals, wherein: each of the first circuits includes afirst transistor and a second transistor connected in series; a gateterminal of the first transistor is coupled to a bias signal differentfrom the outputs of the plurality of storage circuits; and a gateterminal of the second transistor is coupled to the propagated firstsignal.
 2. The system of claim 1, wherein the first transistor includesa bias transistor and the second transistor includes a selecttransistor.
 3. The system of claim 2, further comprising one or moresecond circuits configured to generate a third signal, wherein the oneor more second circuits are connected to the load block to convert thethird signal to a second component of the ramping voltage signal.
 4. Thesystem of claim 3, further comprising one or more comparators eachcoupled to a column read circuit of an image sensor to receive a readoutsignal, wherein the comparator compares the readout signal with theramping voltage signal.
 5. The system of claim 4, further comprising acounter coupled to the comparator, wherein the counter records a valuecorresponding to the ramping voltage signal when the comparator switchesits comparing result.
 6. The system of claim 1, wherein: the pluralityof storage circuits are connected to each other in a series; and a firstof the plurality of storage circuits is configured to receive a rampenable signal.
 7. The system of claim 1, wherein a drain terminal of thesecond transistor is configured to output the second signal.
 8. Thesystem of claim 1, further comprising: a load block coupled to thesecond generating circuits for generating at least a first component ofa ramping voltage signal based on the ramp current signal, wherein aslope of the ramping voltage signal is adjustable based on adjusting atleast one of: the second signal, a resistance of the load block, or alength of the ramping voltage signal.
 9. The system of claim 1, whereinthe first component of the ramping voltage signal has a staircase shapeor a smooth ramp curve.
 10. The system of claim 1, wherein each of thestorage circuits includes a latch, and wherein a first latch receivesthe first signal.
 11. The system of claim 10, wherein the latches aregated D latches or edge triggering registers and the first controlsignal includes a clock signal.
 12. The system of claim 1, wherein eachof the first circuits includes one or more transistors and a gate of onetransistor is coupled to an output of the corresponding storage circuitto receive the propagated first signal.
 13. A system for generating aramping signal, comprising: a plurality of storage circuits eachincluding an input and an output, the output of a previous storagecircuit being connected to the input of a next storage circuit, thestorage circuits being configured to propagate a first signal; aplurality of first circuits, each being coupled to the output of acorresponding storage circuit to receive the propagated first signal andto generate a second signal based at least on the received propagatedfirst signal, the first circuits being configured to obtain a rampcurrent signal based on the generated second signals; and a load blockcoupled to the first circuits for generating at least a part of aramping voltage signal based on the obtained ramp current, wherein aslope of the ramping voltage signal is adjustable based on adjusting atleast one of: the second signal, a resistance of the load block, or alength of the voltage ramping signal.
 14. The system of claim 13,wherein the part of the ramping voltage signal has a staircase shape ora smooth ramp curve.
 15. The system of claim 13, further comprising oneor more second circuits configured to generate a third signal, whereinthe generated part of the ramping voltage is a first component of theramping voltage signal, and the one or more second circuits areconnected to the load block to convert the third signal to a secondcomponent of the ramping voltage signal.
 16. The system of claim 15,wherein: the load block is coupled to the second circuits; and the loadblock is configured to combine currents from the first and secondcircuits to generate the ramping voltage signal.
 17. A system forgenerating a ramping signal, comprising: a plurality of storage circuitseach including an input and an output, the output of a previous storagecircuit being connected to the input of a next storage circuit, thestorage circuits being configured to propagate a first signal; aplurality of first circuits, each being coupled to the output of acorresponding storage circuit to receive the propagated first signal andto generate a second signal based at least on the received propagatedfirst signal, the first circuits being configured to obtain a rampcurrent signal based on the generated second signals; and a load blockcoupled to the first circuits and including one or more resistorsconfigured to convert the obtained ramp current signal to at least afirst component of a ramping voltage signal.
 18. The system of claim 17,further comprising a control circuit block configured to control a totalequivalent resistance of the one or more resistors.
 19. The system ofclaim 17, further comprising one or more second circuits configured togenerate a third signal, wherein the one or more second circuits areconnected to the load block to convert the third signal to a secondcomponent of the ramping voltage signal.
 20. The system of claim 19,wherein: the load block is coupled to the second circuits; and the loadblock is configured to combine currents from the first and secondcircuits to generate the ramping voltage signal.